Field of the Invention
The present invention relates generally to fuel assemblies in nuclear reactors and, more particularly, is concerned with a method of removing failed locking screws, such as ones used to clamp the hold-down leaf springs of the top nozzle, and installing replacement locking screws.
Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device having leaf springs which cooperate with the upper core support plate to prevent the force of the upward coolant flow from lifting the fuel assembly into damaging contact with the upper core plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like.
Heretofore, leaf springs of the aforementioned hold-down device have been fastened and held in their operative positions on the top nozzle by using several corner blocks and a number of screws which extend through the corner blocks and bases of the leaf springs and together with the corner blocks clamp the leaf springs at their respective bases to a peripheral top flange of the top nozzle. In their operative positions, the leaf springs extend in incline upward relationships along and within the outer perimeter of the top nozzle enclosure where they contact the upper core plate. Elongated metal locking pins have been used to retain the spring clamping screws in their tightened down condition. A locking pin is placed in one of the cross grooves on the screw head and the opposite ends of the pin which extend beyond the ends of the groove are welded to portions of the respective corner block adjacent the screw head.
Recently, some fuel assemblies employing the aforementioned hold-down device have been found to have broken leaf spring clamping screws. In at least one case, the broken screws allowed the corner block to shift laterally into the path of the control rod cluster control assembly (RCCA). The RCCA is centrally disposed within the central region of the fuel assembly bounded by the peripheral flange of the top nozzle and controls vertical movement of the control rods into and from the guide thimbles of the fuel assembly. Such inadvertent displacement of the corner block interference with control rod travel.
Evidence points strongly to the cause of the screw failures being stress corrosion cracking in reactor grade water of the high stressed Inconnel-600 material composing the clamping screws. These screw failures can pose serious consequences with respect to the reliability of fuel assemblies employing them and, therefore, as a precautionary measure, the removal of clamping screws of Inconnel-600 material from irradiated fuel assemblies and replacement with screws of different material are warranted.
Since replacement of the prior Inconnel-600 screws with new screws must be performed remotely with the fuel assembly submerged under water at a repair station, a need exists for a method of removing the prior screws with the locking pins welded to the corner blocks and installing new screws and locking the same in the corner blocks without welding the ends of the locking pins to the blocks.